This invention relates to the field of seismic exploration and, more specifically, to a method and apparatus for noise estimation in seismic surveying.
In seismic surveying, acoustic energy waves are transmitted into the earth in order to map subterranean geological structures by sensing returned acoustic energy waves reflected from those geological structures. Land seismic surveys are commonly performed using vibroseis trucks that provide the source of the acoustic energy. The vibroseis trucks generate (xe2x80x9cvibratexe2x80x9d) the acoustic energy waves at predetermined vibrator points (xe2x80x9cVPxe2x80x9d) that are usually marked with stakes that have been placed by surveyors. During operations, the vibroseis trucks navigate from point to point using these survey stakes.
The acoustic energy wave, known as a chirp sweep, vibrated by the trucks is swept in frequency over a period of time. A typical chirp may sweep from 5 to 150 Hertz (Hz) and last for 15 seconds. The subterranean geological layers create changes in the chirp due to refractions, reflections, and diffractions at the boundaries of changes in acoustic impedance of each subterranean layer. Some of these altered acoustic energy waves, known as echoes, return to the earth""s surface to be sensed by seismic detectors. The arrival time of the echoes at the seismic detectors depends mainly on the depth of the subterranean layers reflecting the chirp. A listening time window is used to capture the return echoes down to the depth of interest. The echoes are compressed by correlating with the chirp sweep. The arrival times of the compressed echoes are used to generate imaging data of the subterranean structure.
One problem with seismic surveying operations is that the presence of ambient noise during the listening time window may drown out the echoes to be sensed by the seismic detectors. Ambient noise may be generated by sources in the area being surveyed such as wind tugging on grass or vehicular traffic. The imaging data process requires a minimum signal-to-noise ratio (SNR) for the data to be of sufficient quality for surveying use. If the ambient noise level is too high, then surveying operations may have to be halted until the ambient noise level falls to an acceptable SNR level. As such, in order to obtain an in-field estimate of the data quality, a passive measurement of the ambient noise level is made.
FIG. 1A illustrates a prior art sequential sweep operation that uses a broad band energy detector to measure the total noise energy across the entire frequency band of interest. The broad band noise estimation occurs in between the end of a prior listening region and the start of a new chirp sweep. One problem with such a system is that in order to obtain an accurate noise estimate, a dead time (e.g., 2 seconds shown) when no sweeps occur is necessary for broad band energy detection. The deadtime for noise estimation adds to the overall cycle time of seismic surveying operations.
For example, as illustrated in FIG. 1A, a single chirp sweep (15 seconds), plus listening time (5 seconds), plus noise estimation (2 seconds) may take 22 seconds. If 1000 VPs are made in a day, then approximately 33 minutes are used in a non-productive mode listening for noise. Assuming a 12 hour working day, this translates to approximately 5% of the available work time used for noise estimation.
FIG. 1B illustrates a prior art slip sweep operation that also uses a broad band energy detector to measure noise energy. In slip sweep operations, multiple groups of vibroseis trucks are used in which a group starts sweeping without waiting for the other groups"" sweep to be completed. The sweeps of the different groups occur in non-overlapping frequency-time windows such that no two groups are sweeping in the same frequency at the same time. For example, sweep 2 from a second group is started at time, ts, as soon as the start frequency, fs, of the previous sweep 1""s echo region has completed.
As with sequential sweep operations, a slip sweep operation using a broad band noise estimator requires a dead time (e.g., 2 seconds shown) when no sweeps occur for broad band energy detection in order to obtain an accurate noise estimate. The noise estimate occurs in between the end of the last echo region of a vibroseis group""s sweeps and the start of a new chirp sweep in a new vibroseis group""s sweeps. One problem with using a broad band noise estimator is that the operations of all vibroseis groups must stop in order to obtain the noise estimate, thereby adding to the overall cycle time of seismic surveying operations.
The present invention pertains to a method and apparatus for noise estimation. The method including producing a first chirp signal having a varying frequency over a first time period, sampling noise energy at a frequency different than the first chirp signal frequency during the first time period, and generating a noise estimate based on the noise energy sampled.
Additional features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.